(1) Field of the Invention
The present invention relates to organofunctionalized microporous to mesoporous silica compositions. In particular, the present invention relates to mesostructured silica compositions in which an open framework structure is formed through a high degree of linking between adjacent LSiO3, O3Si—R—siO3, and SiO4 units, where L and R are organofunctional groups.
(2) Description of Related Art
Organofunctional, mesostructured silica compositions have been recognized as useful materials, particularly molecular as sieves, for use as catalysts for a variety of organic chemical transformations, as trapping agents for metal cation contaminants, as adsorbents for the separation of organic molecules, and as film-forming compositions for electronic applications. These hybrid organic-inorganic compositions can be prepared through two conceptually and practically distinct chemical pathways. The first pathway, denoted the “grafting pathway”, is based on the post-synthesis functionalization of a pre-formed pure silica mesostructure. This approach relies on the grafting of organofunctional silanes to the surface silanol groups, denoted ≡SiOH, of the pre-formed mesostructure. Suitable organofunctional silane reagents have at least one hydrolyzable group attached to the organofunctional silicone center.
Theses grafting reagents have the general formula:L4−xSiYx where L is one or more of the desired organofunctional groups and Y is a hydrolyzable group (typically alkoxide or halide) and x is 1, 2, or 3.
The condensation reaction of a surface silanol groups on the surface of the pre-formed molecular sieve with hydrolyzable groups on the grafting reagent couples the functional organo groups to the surface according to the overall reaction represented below to produce an organofunctional surface.
Here ≡Si—OH represents the surface silanol group and
represents the grafting agent.
Depending on the initial silanol group density on the silica surface and the number of hydrolyzable Y groups on the coupling agent, a distribution of one-, two-, or three Si—O—Si covalent linkages are formed between the silica surface and the organofunctional grafting reagent. If only one linkage is formed between the silica surface and the organosilane containing the organofunctional groups, the linkage can be easily broken through hydrolysis reaction upon exposure to water and the organofunctional group is lost. Three linkages to the silica surface avoids loss of the organic functionality through hydrolysis reaction and two bonds would provide intermediate stability. However, even with LSiY3 grafting agents, it is virtually impossible to bind all the organofunctional groups to a silica surface through three Si—O—Si covalent linkages. Such grafting reactions always result in the formation of one, two, as well as three, covalent linkages to the surface of the silica because of spatial mismatches between the surface silonol groups on the surface and the hydrolyzable groups on the coupling reagent. A recent review by Liu et al. (J. Phys. Chem. 104, 8328 (2000)) reveals that the reaction of LSiX3 grafting reagents with mesostructured silica surfaces not only results in fewer than three Si—O—Si linkages to the SiL groups but, in addition, the distribution of the organo groups on the surface is not uniform. Thus, organo-functional molecular sieve silicas formed through coupling reactions with silane grafting reagents are inherently unstable toward hydrolysis and a large portion of the organofunctional groups can be easily dissociated from the surface.
A much more promising approach to forming stable organofunctional mesostructures is to incorporate the organofunctional groups into the molecular sieve framework as it is being assembled from mixtures of organosilicon and silica precursors in the presence of a structure-directing surfactant. In this direct assembly approach the precursor to the organosilicon LSiO3 units is an organosilane reagents of the type LSiY3, where L is the desired organofunctional group and Y is a hydrolyzable group such as an alkoxide or a halogen. The silica precursor to the SiO4 units in the framework is typically a SiY4 species containing four hydrolyzable groups. The Si—OH groups formed through the hydrolysis of Si—Y bonds in the precursors have a sterically favorable chance of undergoing condensation-polymerization reaction to form fully crosslinked Si—O—Si linkages in the framework. Thus, in comparison to the grafting pathway, the direct assembly method should result in optimal framework crosslinking for improved stability, as well as in a more uniform distribution of organofunctional groups for improved access in the framework pores.
There have been several attempts to prepare organofunctional molecular sieve silicas by direct assembly methods from LSiY3 and SiY4 precursors using both ionic surfactants, as well as electrically neutral surfactants as the structure directors (Lim et al., Chem. Mater. 10 467 (1998); Fowler et al., Chem. Commun., 201 (1998); Maquarrie et al., Stud. Surf. Sci, Catal. (2000); Mercier et al., Chem. Mater. 12 188 (2000); Hall et al., Chem. Commun., 201 (1999); Margolese et al., Chem. Mater. 12 2448 (2000); Van Rhijn et al., Chem. Commun. 317 (1998); Richer et al., Chem. Commun. 1775 (1998)). The resulting products all had compositions corresponding to (SiO2)1−x(LSiO1.5)x, when written in surfactant-free, dehydroxylated and anhydrous form. However, x-ray diffraction analysis indicated that the compositions were reliably mesostructured only when x, the fraction of functionalized silicon centers in the framework, was less than about 0.20. At x values greater than 0.20, the long-range mesostructured order was normally lost, as indicated by the absence of an x-ray diffraction peak, thus preventing access to most of the organofunctional groups in the composition. Lim et al. (Chem. Mater. 10, 467 (1998)) represents the only report among many that claims an organofunctional silica mesostructure with an x value somewhat greater than 0.20. In this latter case, the x value was said to be 0.28 when L is a mercaptopropyl group and the structure director is a quaternary ammonium ion surfactant. This is the only organofunctional mesostructure said to have an x value that exceeded the normal limiting value of 0.20 when prepared through direct assembly methods. Even if the claimed value x was accurate, the product was microporous (not mesoporous), and the ionic surfactant was difficult to remove. Thus, there is a need to provide for mesostructured organofunctional (SiO2)1−x(LSiO1.5)x compositions with x values substantially greater than 0.20, regardless of the nature of the organofunctional group L and a process for producing them.
Another approach to the design of organofunctional mesostructures is based on the surfactant-directed assembly of so-called “bis-silyl” mesostructures with compositions corresponding to [O1.5Si—R—SiO1.5] when written in surfactant-free, dehydroxylated and anhydrous form (Inagaki, J. Amer. Chem. Soc. 121 9611 (1999); Aseta et al., Nature 402 867 (1999); Holland et al., 121 4308 (1999)). Here the R group is an organo group that links two tetrahedrally coordinated silicon centers together through silicon-carbon covalent bonds. These mesostructured compositions are prepared through the hydrolysis of a Y3Si—R—SiY3 precursor, where Y is a hydrolyzable group, in the presence of a suitable surfactant. A typical organic linker group is the ethylene group, —CH2CH2—. Most but not all of the oxygen atoms on the O3Si—R—SiO3 units comprising the framework bridge adjacent silicon centers to form a framework around the structure-directing surfactant. Hexagonal, cubic, and wormhole framework structures have been reported. Removal of the structure-directing surfactant by solvent extraction produces an open framework structure with atomically disordered (amorphous) [O1.5Si—R—SiO1.5] walls. Although ethylene linker groups have limited chemical functionality, the mesostructures containing such groups have a lower surface polarity in comparison to pure silica mesostructures and this lower surface polarity is expected to be useful in adsorption applications and molecular separations. One severe limitation, however, is the limited ability to incorporate different organofunctional groups into the framework walls. Other linker groups that have been used to form bis-silyl mesostructures have been limited to phenylene, ferrocenylene, thiophenylene, acetylene, and vinyl groups. Thus, there is a need to provide for new organofunctional bis-silyl mesostructures with surface polarities intermediate between conventional bis-silyl mesostructures and conventional silica mesostructures.
Mercapto-functional mesoporous molecular sieve silicas have received considerable attention as heavy metal ion trapping agents (Feng, S., et al., Science 276, 923 (1997); Chen, X., et al., Separation Sci. Tech. 34 1121 (1999); Mattigod, S. V., et al., Separation Sci. Tech. 34 2329 (1999); Mercier, L., et al., Adv. Mater. 9 500 (1997); Mercier, L., et al., Environ. Sci. Technol. 32 2749 (1998); Mercier, L., et al., Microporous and Mesoporous Mater. 20 101 (1998); Brown, J., et al., Chem. Commun. 69 (1999); Brown, J., et al., Microporous and Mesoporous Mater. 37 41 (2000); Lim, M. H., et al., Chem. Mater. 10 467 (1998); Liu, A. M., et al., Chem. Commun. 1145 (2000)). The anchored thiol groups also can be oxidized to provide sulfonic acid functionality for applications in solid acid catalysis (Lim, M. H., et al., Chem. Mater. 10 467 (1998); Van Rhijin, W. M., et al., Chem. Commun. 317 (1998); Bossaert, W. D., et al., J. Catal. 182 156 (1999); Harmer, M. A., et al., chem. Commun. 1803 (1997); Harmer, M. A., et al., Adv. Mater. 15 1255 (1998); Margolese, D., et al., Chem. Mater. 12 2448 (2000)). The potential usefulness of these derivatives, as well as other organo-functional derivatives, depends critically on the loading of accessible functional groups in the framework. To date, open framework mesostructures have been obtained for compositions in which fewer than 20% of the silicon centers have been functionalized. Therefore, there is a need to devise methods for increasing the loading of mercapto and other functional groups while maintaining the mesoporous framework structure.
Several mercaptopropylsilyl-functionalized mesostructures have been prepared through direct assembly pathways (Lim, M. H., et al., Chem. Mater. 10 467 (1998); Margolese, D., et al., Chem. Mater. 12 2448 (2000); Fowler, C. E., et al., Chem. Commun. 1769 (1997); Hall, S. R., et al., Chem. Commun. 201 (1999), as well as through grafting reactions of preassembled frameworks (Feng, S., et al., Science 276 923 (1997); Mattigod, S. V., et al., Separation Sci. Tech. 34 2329 (1999); Mercier, L., et al., Adv. Mater. 9 500 (1997); Mercier, L., et al., Environ. Sci. Technol. 32 2749 (1998); and Liu, A. M., et al., Chem. Commun. 1145 (2000)) using 3-mercaptopropyltrimethoxysilane (MPTMS) as the funtionalizing agent. In general, as discussed, direct assembly pathways are preferred over grafting methods, in part, because direct assembly pathways afford a more uniform distribution of organo groups on the framework walls. Also, direct assembly allows for better crosslinking of the silane moiety to the silica framework. For instance, 20–30% of the framework silicon centers in hexagonal SBA-15 (Margolese, D., et al., Chem. Mater. 12 2448 (2000) and MCM-41 (Lim, M. H., et al., Chem. Mater. 10 467 (1998)) mesostructures, have been funtionalized with mercaptopropyl groups through the direct assembly of MPTMS and tetraethylorthosilicate (TEOS) mixtures. Also, mercaptopropyl-functionalized silicas with wormhole framework structures, denoted MP-HMS, have been prepared through analogous assembly methods (Brown, J., et al., Chem. Commun. 69 (1999); Macquarrie, D. J., et al., Stud. Surf. Sci. Catal. (2000); Mercier, L., et al., Chem. Mater. 12 188 (2000)).
When assembled from highly polar solvents, organofunctional mesostructures can form sponge-like particles through the intergrowth of mesoscopic wormhole framework domains. Consequently, the framework sites of HMS silicas are generally more accessible for metal ion trapping and chemical catalysis in comparison to their hexagonal SBA-15 and MCM-41 counterparts with the same framework pore size, but with highly monolithic particle morphologies. On the basis of earlier literature reports (Brown, J., et al., Chem. Commun. 69 (1999); Macquarrie, D. J., et al., Stud. Surf. Sci. Catal. (2000); Mercier, L., et al., Chem. Mater. 12 188 (2000)); however, the direct assembly of HMS mesostructures appears to limit the degree to which organo groups functionalize the framework. Little or no mesostructure formation was realized, for instance, at MP loadings above about 20 mole % (Macquarrie, D. J., et al., Stud. Surf. Sci. Catal. (2000)).
Relevant patent art on silica compositions includes U.S. Pat. Nos. 5,672,556, 5,712,402, 5,785,946, 5,800,800, 5,840,264, 5,855,864, 6,193,943 and 6,162,414 to Pinnavaia et al. Organofunctionalized silicas are described in PCT WO 01/12564 all owned by a common assignee.